To preserve his own space, he said, he once gave a young girl $5 (with her mother's permission) in exchange for not reclining her seat, an original solution.

If you don't want to open your wallet, make sure to look behind you before you recline. Maybe warn the fellow traveler whose space you are about to invade, so that person can hang onto his or her drink or adjust his or her laptop.

And stay upright during meals.

For those who opt for the Knee Defender and manage to skirt the watchful eyes of flight attendants, we think it's appropriate to warn the passenger ahead of you, or at least hand him or her the courtesy card.

Here's a 2007 video produced by FareCompare (featuring an elf and Santa Clause) showing how the Knee Defender works: